1. Field of the Invention
The present invention relates to a current detection device of a so-called magnetic balance type including a feedback coil.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2013-53903 describes an invention related to a current detection device of a so-called magnetic balance type.
In this current detection device, a magnetoresistive element and a feedback coil face a conductor through which a current to be measured passes. A current magnetic field excited by the current that is to be measured and that flows through the conductor is detected by the magnetoresistive element, and control is performed so that a feedback current corresponding to the magnitude of the detection output is applied to the feedback coil. A cancelling magnetic field, which is reverse to the current magnetic field, is applied from the feedback coil to the magnetoresistive element. When the current magnetic field and the cancelling magnetic field reach a balanced state, the current flowing through the feedback coil is detected, and the detection output of the current is obtained as a measured value of the current.
As illustrated in FIG. 4, in the current detection device described in Japanese Unexamined Patent Application Publication No. 2013-53903, a shield layer is disposed between the conductor through which a current to be measured flows and the feedback coil. The shield layer weakens the current magnetic field induced by the current to be measured, and the weakened current magnetic field is applied to the magnetoresistive element. Accordingly, the range of intensity of the current to be measured that can be detected by the magnetoresistive element is widened, and the dynamic range for measuring the current magnetic field can be expanded.
To manufacture the current detection device described in Japanese Unexamined Patent Application Publication No. 2013-53903, it is necessary to laminate, on a substrate, a magnetoresistive element, a lower insulating layer that covers the magnetoresistive element, a feedback coil located on the lower insulating layer, and an upper insulating layer that covers the feedback coil in this order, and to form a shield layer on the upper insulating layer in a plating process so as to cover the feedback coil.
The upper insulating layer that covers the feedback coil may be an organic insulating layer. However, the organic insulating layer, which is hygroscopic, may deteriorate the feedback coil and the shield layer that are in contact with the upper insulating layer. In addition, since the organic insulating layer swells by absorbing water, stress is applied to the magnetoresistive element, the feedback coil, and so forth, and accordingly the bonding strength at the boundary between the upper insulating layer and the shield layer is more likely to decrease.
For this reason, it is preferable to form the upper insulating layer by using an inorganic material, such as Si-Nx. However, if the insulating layer made of an inorganic material is formed by using chemical vapor deposition (CVD) or spattering, relatively large stepped portions are inevitably produced at a surface of the upper insulating layer, from an upper surface of a coil layer constituting the feedback coil to both outer sides of the coil layer at both side portions of the feedback coil, because the coil layer has a relatively large height. The shield layer needs to have a width for covering the feedback coil, and thus the shield layer is formed to cover the stepped portions at the surface of the upper insulating layer at both side portions of the shield layer.
A process of manufacturing a current detection device includes a heating process, such as a process of firing a resin for a package, after a shield layer has been formed, and also includes another heating process of soldering the current detection device that has been completed to a mother substrate. There is a large difference in linear expansion coefficient between the upper insulating layer made of an inorganic material and the shield layer made of a metallic material by using plating. Thus, heat stress is likely to affect the boundary between the upper insulating layer and the shield layer during a cooling process after each heating process.
As described above, if the upper insulating layer is formed by using an inorganic material, stepped portions are likely to be produced in the upper insulating layer at both side portions of the feedback coil, and the shield layer is superimposed on the stepped portion while being deformed. In such a multilayer structure, the heat stress between the upper insulating layer and the shield layer concentrates on the stepped portion of both the layers, and the concentration of stress is likely to cause a crack at the stepped portions of the upper insulating layer.
In addition, if stepped portions are formed at the surface of the upper insulating layer, step-like deformed portions are formed also on both sides of the shield layer. The deformed portions formed on both sides of the shield layer decreases an anisotropic magnetic field Hk in the width direction of the shield layer, that is, in the sensitivity-axis direction of the magnetoresistive element, and saturated magnetization in the same direction of the shield layer decreases. As a result, a shield effect decreases and the dynamic range of a current to be measured becomes narrow.